1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus adapted to correct errors of phase encoding amounts in a split scan of a high-speed spin-echo imaging technique or the like.
2. Description of the Related Art
In a split scan of a high-speed spin-echo imaging technique performed by magnetic resonance imaging apparatus (MRI apparatus), an operation of exciting nuclear spins once by a 90-degree-flip radio-frequency (RF) excitation pulse to generate a plurality of echo signals (MR signals) and applying a predetermined phase encoding amount to each of the echo signals is repeated a plurality of times. Thereby, a plurality of echo signals of varying phase encoding amounts, which are required to reconstruct a-single MR image, are acquired.
The phase encoding amount for each echo signal will involve an error when at least one of the offset value of a gradient magnetic field pulse Ge, the duration of the pulse Ge and an increment in phase encoding amount between encoding steps deviates from their respective predetermined values. Errors of phase encoding amounts will produce image artifacts on a reconstructed image. Conventionally, the following methods have been used to decrease or correct such errors of phase encoding amounts.
According to a first method, gaps between every split region are eliminated and phase encoding amounts are set so that a plurality of echo signals are continuous on the k space (Fourier space). According to a second method, phase encoding amounts are gradually adjusted so that image artifacts on a reconstructed image will disappear. According to a third method, the output characteristics of units for generating gradient magnetic fields are measured and control signals to the units are adjusted on the basis of the results of the measurement.
With the first method, however, even if the continuity of echo signals is maintained in connection portions on the k space, errors of phase encoding amounts cannot be detected in portions other than the connection portions. In addition, even when normal phase encoding is not performed, the possibility exists that the continuity of echo signals may be maintained in the connection portions by accident.
With the second method, it is difficult to find an adjustment element that permits image artifacts to disappear.
The third method increases the possibility that predetermined phase encoding amounts cannot be obtained because of the insufficiency of gradient field generating power supplies and inhomogeneities in the magnetic field caused by eddy currents. In addition, with the third method, phase encoding amounts cannot be adjusted after installation.